Odor sensor with organic transistor circuitry

ABSTRACT

Circuits include at least one-odor sensitive organic transistor having a conduction channel whose conductivity changes in response to certain odors. The organic transistors are interconnected to increase their response to selected odor signals. The organic transistors may be interconnected to form a ring oscillator whose frequency of oscillation changes in response to an odor signal and in which the alternating signal applied to the gate electrodes of the organic transistors enhances their recovery and reduces their drift.

BACKGROUND OF THE INVENTION

This invention relates to circuitry employing organic transistors and,in particular, organic field effect transistors (OFETs) to detectchemical odors/vapors/gases (analytes).

Many different types of OFETs are known. By way of example, FIG. 1 showsthe structure of an OFET 10 having a semiconductor body region 12 with asource electrode 14 and a drain electrode 16 defining the ends of aconduction channel through the semiconductor body 12. The OFET 10 alsoincludes an insulator layer 18 and a gate (control) electrode 20 towhich a voltage may be applied to control the conductivity of thesemiconductor body region (i.e., the conduction channel). The OFET ofFIG. 1 is manufactured to have organic material in its semiconductorbody region 12 that can absorb analytes and which, in response to theabsorbed analytes, changes the conductivity characteristics of theconduction channel. As illustrated in FIG. 1, analytes(vapors/odors/gases) may flow over the OFET for a period of time.Ensuing changes in the conductivity of the OFET may be measured as shownin FIG. 1A by sensing the current (I_(d)).

In known circuitry, the OFETs have been used as discrete devices. Asshown in FIG. 1A, the source of an OFET may be connected to a firstpoint of operating potential (e.g., VDD) and its drain may be connectedvia a load resistor RL to a second point of operating potential (e.g.,ground potential). The gate of the OFET may be biased via resistors R1and R2 to produce a desired operating direct current (d.c.) bias levelwithin the source-drain (i.e., conduction) path of the OFET. The OFETmay then be subjected to a flow of analytes which causes itsconductivity to change. The corresponding change in conductivity of theOFET is then detectable by a circuit connected to the drain and/or thesource of the OFET.

A problem with known OFETs is that their sensitivity to the analytes isrelatively low. Also, known OFETs are subject to drift and thresholdshift as a function of time, as shown in FIG. 2A and FIG. 2B,respectively. In FIGS. 2A and 2B, it is seen that, for a fixed biascondition, source-to-drain current (I_(d)) of an OFET changes (e.g.,decreases) as a function of time. This is the case when there is nosignal input (i.e., no odor), as illustrated by waveform A of FIG. 2Aand waveform portion C in FIG. 2B. This is also the case following theapplication of an odor to the OFET, as illustrated in waveform B of FIG.2A and in waveform portion D in FIG. 2B. That is, for a fixed biascondition, the current through the conduction path of the OFET changes(drifts) as a function of time. OFETs may also be subjected tohysteresis and offsets. As a result of these characteristics, it isdifficult to use OFETs in known discrete circuits to differentiate aninput signal from background conditions and to determine or measure thefull extent of the input signal.

SUMMARY

Problems associated with the characteristics of OFETs, such astime-related drift, detract from their use as sensors and amplifiers oftheir sensed signals when the OFETs are used as discrete devices.Applicants recognized that OFETs should be incorporated in circuitsspecifically designed to overcome and/or cancel the problems associatedwith certain characteristics of OFETs such as their drift, thresholdshift and hysteresis.

Circuits of various embodiments include at least one odor-sensitiveorganic transistor having a conduction channel whose conductivitychanges in response to certain ambient odors.

In one embodiment, organic transistors are interconnected to increasetheir response to selected odor signals and such that the recovery ofthe organic transistors is enhanced and their drift is reduced. In aparticular embodiment, the organic transistors are interconnected toform a ring oscillator whose frequency of oscillation changes sharply inresponse to an odor signal and in which the alternating signal appliedto the gate electrodes of the organic transistors enhances theirrecovery and reduces their drift.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawing, like reference characters denote likecomponents; and

FIG. 1 is a highly simplified cross-section of a known OFET suitable foruse in various embodiments;

FIG. 1A is a prior art circuit diagram of a discrete OFET circuit tosense and amplify signals resulting from odors;

FIGS. 2A and 2B are diagrams illustrating the drift in the currentthrough the conduction path of an OFET as a function of time;

FIGS. 3A and 3B are the symbolic representation of P-type OFETs andP-type FETs, respectively, used in this application;

FIGS. 4A and 4B are the symbolic representation of N-type OFETs andN-type FETs, respectively, used in this application;

FIG. 5 is a schematic diagram of an OFET-differential amplifiercombination embodiment the invention;

FIG. 6 is a schematic diagram of an OFET- amplifier combinationembodiment;

FIG. 7 is a block diagram of a system employing OFETs in one embodiment;

FIG. 8 is a schematic diagram of a ring oscillator circuit employingOFETs in one embodiment;

FIG. 9 is a diagram of waveforms associated with one embodiment of thecircuit of FIG. 8;

FIG. 10 is a schematic diagram of another ring oscillator circuitemploying OFETs in one embodiment;

FIGS. 10A and 10B are schematic diagrams of cascaded inverters usingOFETs for analyte sensing;

FIG. 11 is a block diagram of an odor sensor in one embodiment;

FIG. 12 is a cross section of an OFET suitable for use in circuitsembodying the invention;

FIG. 13 is a drawing of various molecular structures of severalmaterials used to make OFETs; and

FIG. 14 is a drawing of examples of molecular structures of odors to besensed by circuits of various embodiments.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the discussion to follow, reference is made to organic field effecttransistors (OFETs) which may be used to sense odors, vapors, chemicalsand/or gases (analytes). These terms are used interchangeably and areintended to include each other in the specification and in the claimsappended hereto. OFETs have been described in the literature and theteachings of these references as to the manufacture of these transistorsand their reported characteristics are incorporated herein by reference.In the discussion to follow references to OFETs will also includeorganic thin film transistors (OTFTs) and any device having similarcharacteristics.

To better understand the discussion to follow and the drawings appendedhereto, certain characteristics of OFETs will first be discussed. OFETs(or OTFTs) may be of N-conductivity type or P-conductivity type. To moreeasily differentiate the OFETs from known field effect transistors(FETs), the following symbology will be used in the appended drawings:(a) P-type OFETs will be shown as illustrated in FIG. 3A; (b) P-typeFETs will be shown as illustrated in FIG. 3B; (c) N-type OFETs will beidentified as shown in FIG. 4A; and (d) N-type FETs will be identifiedas shown in FIG. 4B. The drawings for the OFETs include a rectangularbox, with an X through the box, indicative of the semiconductor bodywith source and drain electrodes attached to the semiconductor bodyrepresenting the ends of a conduction path (or channel) through thesemiconductor body. The semiconductor body is electrically insulatedfrom a gate (control) electrode, g, to which a bias voltage or a signalmay be applied to control the conductivity of the semiconductor body. AP-type OFET is shown with an arrow pointing towards the body of the OFETand an N-type OFET is shown with an arrow pointing away from the body ofthe OFET. The semiconductor body defines the conduction path and theOFET includes source and drain electrodes defining the ends of theconduction path. OFETs, like FETs, are generally bi-directionalconducting devices. Therefore: (a) the source of a P-type OFET or of aP-FET is defined as the one of the two electrodes connected to thesemiconductor body whose potential is more positive than the other(drain) electrode; and (b) the source of an N-type OFET or of an N-FETis defined as the one of the two electrodes connected to thesemiconductor body whose potential is less positive than the other(drain) electrode.

As noted above, OFETs are typically subject to several problems:

a) low sensitivity; (b) drift; (c) hysteresis; and (d) variation oftheir threshold voltage (V_(T)) as a function of time.

The various embodiments are directed to circuits that use OFETs but areconfigured to reduce or compensate for the problems associated with lowsensitivity, drift, threshold shift and hysteresis.

FIG. 5 illustrates the use of an organic transistor, OFET M2, as asensor of odors and as an amplifier of the signal sensed. In thediscussion to follow, when an odor flows over an OFET and the OFET isturned-on and/or biased into conduction, the OFET may be said to be“sensing” or “sniffing” the odor and to be in a sensing or sniffingmode. OFET M2 forms part of the input stage of a differential amplifier.The amplifier includes selectively enabled adaptive feedback circuitry,which enables the background, drift and threshold shift conditions to beeffectively subtracted from the amplified output signal. When theamplifier circuit is in a standby non-sensing mode (i.e., not sensing orsniffing any odors, gases, chemical vapors etc.) negative feedback isused to cancel any drift or threshold shift due to OFET M2. When thecircuit is in a sensing mode (i.e., “sniffing”) the feedback loop isopened with the circuit biased in a high gain state so it can respondquickly and with high gain to an input signal.

In the circuit of FIG. 5, transistor M2 is an odor-sensitive P-typeOFET. For ease of illustration the other transistors used in the circuitare non-odor sensitive FETs or OFETs. Transistors M1, M2, and M5 formthe input stage of a differential amplifier, with the source electrodesof M1 and M2 being connected to the drain of M5. Thus, M1 and M2 competefor the current from current-source M5. The source electrode of M5 isconnected to a point of fixed operating potential (i.e., VDD) and a biasvoltage V_(B) is applied to the gate of M5 causing a constant current,Io, to flow through the conduction path of M5. The current Io is equalto the sum of the current I1 through the conduction path of M1 and thecurrent I2 through the conduction path of M2; that is, Io=I1+I2. Thevalue of the current flowing through the conduction paths of M1 and M2is a function of their relative gate voltages. The lower the gatevoltage of M1 with respect to M2, the greater is the fraction of thecurrent Io that flows through it; similarly for M2. The drain of M1 isconnected to node A1 and the drain of M2 is connected to node A2.

The current I1 flowing into node A1 is mirrored via a current mirrorcircuit comprised of transistors M3 and M6 to produce a current I6flowing through the conduction paths of transistors M6 and M8. For easeof illustration, assume that the current current I6 which is equal tothe current I8 is also equal to the current I1 ; i.e., I6=I8=I1. Thecurrent through M8 is then mirrored via a current mirror comprised oftransistors M8 and M9 to cause a current I9 to flow through theconduction path of M9 into node A3. The sources of M8 and M9 areconnected to VDD volts and their gates are connected in common to thedrains of M8 and M6. When M8 and M9 have similar geometries, their draincurrents will be substantially equal for the same gate-source biasconditions. In that case, the current I9 flowing into node A3 is equalto I6 which is, in turn, equal to I1; i.e., I1=I6=I9.

The drain current I2 through M2 flows into node A2 and is mirrored viathe current mirror transistor combination of M4 and M7. The sources ofM4 and M7 are connected to ground potential and the gate of M7 isconnected to the gate and drain of M4. When M4 and M7 are of similargeometries their drain currents will be substantially the same for likegate-source bias conditions. Thus, M4 and M7 function as a currentmirror to produce a current I7 through the conduction path of M7 whichis drawn out of node A3. In that case, the current I7 is equal to thecurrent I2. For the above conditions, if I1 is equal to I2, the currentI9 flowing into node A3 is equal to the current I7 flowing out of nodeA3. Where M9 and M7 have essentially equal impedances for thecorresponding bias condition, the voltage at node A3 will besubstantially equal to VDD/2 when I1 is equal to I2. It should also beappreciated that, since M7 and M9 are effectively high impedance currentsources, a small difference between the currents I9 and I7 results in alarge voltage differential at node A3. Thus, the circuit has a verylarge open circuit (i.e., when there is no feedback) voltage gain.

Thus, the pair of currents I1 and I2 are mirrored via the currentmirrors formed by M3-M6, M4-M7, and M8-M9 and compared with each otherto generate an output voltage at node A3. If the current through M2exceeds that through M1, then the node voltage A3 is driven to a valuenear ground. If the current through M1 exceeds that through M2, then thenode voltage A3 is driven to a value near VDD volts. Thus, transistorsM1-M9 implement a wide-output range differential amplifier with inputsgiven by the gate voltages of M1 and M2 and an output given by thevoltage at node A3.

The gate of OFET M2 is connected to a relatively constant bias voltagesource V2. To better illustrate the operation of the circuit and therole of M2 as a sensor, an input signal source 71 is shown connectedbetween the source and gate of M2. The source 71 and its connections areshown with dashed lines since this source is internal to OFET M2. TheVin source 71 depicts the equivalent input signal due to a mobility orthreshold voltage change in transistor M2 when an odor is “puffed” onto(i.e., applied to) it. An odor “puffed” onto transistor M2 is equivalentto the application of an input signal to its gate. When an odor to besensed is applied to M2, the feedback loop is opened (i.e., P-typetransistor M12 is turned off by driving voltage V12 to an active highstate). Transistor M2 is the only odor sensitive transistor in thecircuit of FIG. 5.

The gate of transistor M1 is connected to the output (i.e., node A5) ofa low pass filter whose input is connected to the output (i.e., node A3)of the differential amplifier to provide a negative feedbackconfiguration. That is, output node A3 of the amplifier is applied tothe input of a source follower stage (i.e., the gate of M10) comprisedof transistors M10 and M11. The source of M11 is at VDD volts and a biasvoltage V11 is applied to the gate of M11 to establish a current throughM11 and M10. The source of M10 is connected to the drain of M11 at anoutput node A4 and the drain of M10 is returned to ground potential. Theconduction path of a transistor switch M12 is connected between node A4and node A5 which is connected to the gate of M1. A capacitor C1 isconnected between the gate of M1 and ground potential. The low passfilter is implemented with transistors M10, M11 and capacitor C1. Whentransistor M12 is turned on closing the feedback loop (i.e. the circuitis not “sniffing”), the output at node A4 of the source follower isconnected to capacitor C1 (via the low “ON” impedance of M12) such thattransistors M10, M11 and capacitor C1 implement a weakly nonlinear lowpass filter. The time constant of the low pass filter may be controlledby altering V11 or the value of the capacitance of C1 or both.

When the feedback loop is closed (i.e., M12 is turned on), any drift orchange in the conductivity of M12 is effectively cancelled because aconductivity change of M2 causes a corresponding change in I2. Thechange in I2 then causes a corresponding change in the voltages at nodesA3 and A4. The change at A4 is then applied via M12 to the gate of M1with a magnitude and polarity to cause a change in I1 which cancels oroffsets the change in I2 caused by M2 (i.e., negative feedback tends tocause I1 to equal I2). Thus, when the negative feedback loop is closed,the gate voltage of M1 adapts to compensate (or cancel) for long-term,time dependent, changes between the threshold or mobility of transistorsM1 and M2 and automatically keeps the differential amplifier output atits balanced midpoint.

As noted above, when the circuit is not sniffing, the negative feedbackis turned on and causes the circuit to adapt and compensate for anylong-term differences between the transistor characteristics of M1 andM2. The negative feedback is turned on by causing V12 to go low (e.g., 0volts) and transistor M12 to be turned on. When M12 is turned on, thefeedback voltage applied to the gate of M1 causes the currents I1 and I2to be substantially equal. Assume that M3, M4, M6 and M7 are all made tothe have the same geometry. Then, for I1 equal to I2, the current I2 ismirrored through M7 so a current equal to I2 is drawn from node A3.Concurrently, the current I1 is mirrored through M6 and M8 and thenmirrored via M9 to produce a current equal to I1 flowing into node A3.For I1 equal to I2, the voltage at node A3 will be equal toapproximately VDD/2. The voltage at node A3 is applied to the input ofsource follower stage M10 to produce a similar output at node A4. WhenM12 is turned on, the voltage at A4 is applied, via the conduction pathof M12, to the gate of M1 and will tend to cause equal currents to flowthrough M1 and M2. The high degree of feedback when M12 is turned onensures that any drift in M2 gets compensated (i.e., the effect of thedrift is effectively cancelled). Thus, when the circuit is ready to beused to sense (“sniff”) the presence of any vapors or chemicals,transistor M12 is turned off by the application of a high voltage (e.g.,VDD) to the gate of M12. When that occurs, the circuit is biased at anoptimal operating point to respond to signals resulting from odors being“puffed” (applied) to the sensing OFET M2.

When the circuit is about to sniff or is sniffing, the feedback loop isopened (i.e., M12 is turned off) and the circuit sits at its high-gainbalanced midpoint, ready to amplify any odor-caused change in thecurrent through M2. The voltage at A3 serves as the output of thecircuit with changes in the voltage of A3 reflecting changes induced bythe odor response of M2.

This is best illustrated as follows. When the circuit is ready to sniffthe presence of an odor, the feedback loop is opened (i.e., transistorM12 is turned off). When the loop is opened, capacitor C1 is charged(and remains so for some time) to the voltage present at the output ofthe amplifier immediately before M12 was turned off. Thus, the gatevoltage of M1 which represents one of the two inputs of the differentialamplifier is held at a value representative of the gate voltage justbefore the feedback loop is opened. When the odor is applied, M2responds and its conductivity is modified by the chemicals present inthe air or vapor being “sniffed”. If the conductivity of M2 is decreasedby the “input signal” then the current I2 is decreased relative to thecurrent I1 and the voltage at node A3 will rise sharply and quickly inresponse thereto. On the other hand, if the conductivity of M2 isincreased by the “input signal” then the current I2 is increasedrelative to the current I1 and the voltage at node A3 will drop sharplyand quickly in response thereto. In either case a good indication of theinput signal condition will be produced at the output of the amplifierwith the d.c. shift and drift substantially removed from the outputsignal.

FIG. 5 also shows a switching subcircuit formed by transistors M12, M13,and M14. The implementation of the low pass filter and the switchingsubcircuit are now briefly described. Transistor M12 implements a switchthat is turned on when the voltage V12 is low. Normally, V12 is drivenlow when the circuit is not “sniffing” and is driven high when thecircuit is “sniffing”. The conduction channels of transistors M13 andM14 may be ratioed to have half the width (W) and the same length (L) astransistor M12. They help to alleviate charge injection problems causedby the switching voltage of M12. The charge injection is alleviated byhaving a signal complementary to V12 drive M13 and M14. The chargeinjection is dominated by the overlap capacitances of transistor M12;the overlap capacitances of shorted-and-ratioed transistors M13 and M14match those at the source and drain ends of M12 and serve to cancel theeffects of positive charge injection from M12 with negative chargeinjection from M13 and M14.

FIG. 6 shows a circuit in which an odor responsive P-type OFET M21 isinterconnected with a transistor M11 to form a common-source amplifier.The output (node A31) of the common-source amplifier is connected to theinput of a source (voltage) follower stage comprised of transistors M31and M41 whose output (node A41) is selectively fed back to the gate ofM11 via transistor M121. As in FIG. 5, when odors/vapors are “puffed”onto OFET M21 the feedback loop is opened, and the common-sourceamplifier amplifies the signal due to the odors/vapors.

In FIG. 6, the source of M21 is connected to a power terminal 81 towhich is applied VDD volts and its gate is connected to a constant biasvoltage V21. The drain of M21 is connected to the drain of an N-typeFET, M11, at output node A31. The source of M11 is connected to ground.Node A31 is connected to the gate of source follower transistor M31whose drain is connected to terminal 81 and whose source is connected toterminal A41. Transistors M11 and M21 form a common-source amplifierwith a control input being the gate voltage of M11 and a signal inputbeing the current through M21 responsive to the odors/vapors puffed ontoM21. The output of the common-source amplifier is the voltage at nodeA31. If the current through M21 exceeds the current through M11, thenthe node voltage A41 is driven near VDD. If, on the other hand, thecurrent through M11 exceeds that through M21, the node voltage A41 isdriven near ground. The output of the common-source amplifier isconnected to the input of a source follower stage (the gate of M31)whose output (A41 at the source of M31) is fed back to the gate of M11via switching transistor M121.

As in FIG. 5, an input signal source 71 a is shown (with dashed lines)connected between the gate and source of M21 to indicate the signalinput function of the sensor, internal to M21, when an odor/vapor ispuffed onto M21. That is, signal source 71 a represents the effect of amobility or threshold change on and within transistor M21 when an odoris puffed onto it. Typically, the odor is puffed onto transistor M21when the feedback loop is open (i.e., M121 is turned-off). TransistorM21 is the only odor sensitive transistor in the circuit; any otherorganic transistor in the circuit is assumed to be odor insensitive.

Transistors M31 and M41 form a standard N-type FET source follower stagewhose bias current is set by a voltage V41 applied to the gate of M41.Transistor M121 is turned on when the circuit is not sniffing. When M121is turned on the output of the source follower is tied to a capacitorC11 such that transistors M31, M41 and the capacitor C11 implement aweakly nonlinear low pass filter. The time constant of the low passfilter may be controlled by altering V41, the capacitance of C11, orboth. Transistor M121 implements a switch that is turned on and off by asignal source 121 producing a voltage V121.

When the circuit is not sniffing, the source 121 applies a low voltageto the gate of M121 to enable the negative feedback loop and cause thevoltage at A41 to be applied to capacitor C11 and the gate of M11. Thus,during the non-sensing mode, the gate of transistor M11 is connected toa low pass filtered version of the common-source amplifier's output in anegative feedback configuration. Consequently, during this mode, thegate voltage of M11 constantly adapts to compensate for long-termchanges in the threshold or mobility of transistor M21 and keeps theoutput A31 of common-source amplifier (M11, M21) at its balancedequilibrium.

When the circuit is sniffing, the negative feedback is turned off andthe circuit sits at its gain balanced equilibrium, ready to amplify anyodor-caused change in the current through transistor M21. The outputvoltage at A31 reflects changes induced by the odor response of M21.When the circuit is not sniffing, the negative feedback is turned on andcauses the circuit to adapt and compensate for any long-term changes inthe characteristics of OFET transistor M21.

FIG. 6 also shows a switching sub-circuit formed by transistors M121,M61, and M71 which is active only when V121 is active low. Theimplementation of the lowpass filter and the switching subcircuit arenow briefly described. Transistors M61 and M71 are ratioed to have halfthe W and the same L as transistor M121. They help to alleviate chargeinjection problems caused by the switching voltage of M121. The chargeinjection is alleviated by having a signal (V13) complementary to V121drive M61 and M71. The charge injection is dominated by the overlapcapacitances of transistor M121. The overlap capacitances ofshorted-and-ratioed M61 and M71 match those at the source and drain endsof M121 and cancel charge injection from M121 with charge injection fromM61 and M71.

Features of the circuits of various embodiments, which were discussedabove in FIGS. 5 and 6, are shown in FIG. 7. FIG. 7 includes a high gainamplifier 91 responsive to signals from an OFET sensor that is integralto one of the amplifying devices in amplifier 91. The output of theamplifier is selectively fed back by means of a switching network 92 anda low pass filter 93 to a control input of the amplifier 91. Theswitching network is turned on and off as a function of a sniff signalcircuit 94 which controls the application of chemical odors/vapors/gases(analytes) to the sensor contained within the high gain amplifier.During a non-odor-sensing period of time, the switching network 92closes the negative feedback loop such that the low pass filter 93 iscoupled between the output of amplifier 91 and an input of amplifier 91.During an odor-sensing period of time, the switching network is open sothat the feed back loop is removed from the circuit, and the high gainamplifier 91 amplifies signals resulting from the flow of odors over theOFET sensor.

To better explain other embodiments, assume, as shown in FIGS. 1A and2B, that a discrete OFET is biased to conduct a source-drain currenthaving a value I_(D1) prior to any odor being applied to the OFET, Whenan odor is “puffed” onto the OFET, the source-drain current changes fromI_(D1) to I_(D2) in response to an odor (analyte) incident on the OFETfrom a time I1 to a time I2. Thus, after an odor signal is applied to anOFET, the source-to-drain current changes. The change in thesource-to-drain current persists even after the removal of the odor.When an odor is applied for a given time (e.g., 5 seconds), it normallytakes a much longer period of time (e.g., one minute) for the OFETcurrent to recover from the value of I_(D2) to a value approximatelyequal to that of I_(D1)

After an OFET is subjected to an odor signal, applying an electricalbias cycle to the gate of an OFET facilitates its recovery to thecondition existing prior to the application of the odor. That is, byapplying an electrical signal to the gate of the OFET which goespositive and negative (or negative and positive) relative to the source(and/or drain) of the OFET, the OFET recovers more quickly and thedegree of recovery is enhanced. Full recovery means the return of thedrain current of the OFET to the level it would have had had an odorsignal not been applied to the OFET.

In various embodiments, OFETs are operated so that they return morequickly to the existing operating condition extent immediately beforethe application of the selected odor (analyte) to the circuit. Ringoscillators employing OFETs to sense the presence of odors are veryuseful as sensing circuits for selected odors.

Ring Oscillator Sensors

Using OFETs in a ring oscillator circuit eliminates many of theabove-discussed problems associated with OFETs. Referring to FIG. 8there is shown five complementary inverters (I1-I5) interconnected toform a ring oscillator. In FIG. 8 each inverter (I1-I5) includes aP-type OFET (P1-P5) and an N-type FET (N1-N5). In one embodiment of theFIG. 8 circuit, the P-type OFETs were made of didodecyl α-sexithiophene(DDα6T) material and the N-type FETs were made of hexadecafluoro copperphthallocyanine (F₁₆CuPc) material. In that embodiment, the materialDDα6T was used to form the P-type OFETs because DDα6T is sensitive tothe analyte octanol and esters such as allyl propionate which are theanalytes selected to be sensed by the circuit. In contrast, the materialF₁₆CuPc was used to make the N-type OFETs, because it is insensitive tooctanol and esters such as allyl propionate. Therefore, the N-type OFETs(N1-N5) are insensitive or, at least, less sensitive to the selectedanalytes and could be replaced by standard N-type FETs.

The source electrodes of the P-type transistors (P1-P5) are connected toa power terminal 81 to which is applied +VDD volts. The sourceelectrodes of the N-type FETS (N1-N5) are connected to a power terminal83 to which is applied ground potential. The gate electrodes of the twotransistors forming each inverter are connected in common and define asignal input terminal to the inverter. The drain electrodes of the twotransistors forming each inverter are connected in common and define asignal output terminal of the inverter. Starting with the firstinverter, the output of each inverter along the chain is connected tothe input of the next inverter along the chain, except for the output ofthe last inverter (e.g., I5) which is fed back to the input of the firstinverter. Note that there is some capacitance, C, (which may beparasitic or discrete) associated with the input (gates) of eachinverter. The combination of the effective output impedance of eachinverter and the input capacitance of the next stage determines the timeconstants of each stage and the frequency of oscillation of the circuit.

In one embodiment, the oscillation frequency of the 5-stage all organicF₁₆CuPc/DDα6T complementary ring oscillators ranged from a few Hz toseveral kHz. A selected analyte was “puffed” onto the ring oscillatorcircuit. The analyte reduced the conductivity of the P-type OFETs. Inthe discussion to follow, it is assumed that the conductivity of theOFET decreases when subjected to a gas (analyte). However, it should beunderstood that other OFETs have conductivities that increase whensubjected to an odor (analyte). For OFETs whose conductivity increasesin response to the presence of an odor, the circuit configurationsdiscussed are also suitable. However, the response of the circuit wouldbe the inverse of that described below (i.e., the frequency ofoscillation would increase rather than decrease).

The mobility of the discrete F₁₆CuPc transistors and of the DDα6Ttransistors, measured on devices fabricated alongside the ringoscillators, was approximately equal to ˜10⁻² cm²/V-s. The response ofthe circuit was measured with an oscilloscope with a high inputimpedance (50 M ohm) probe. The response of a particular circuitconfigured as illustrated in FIG. 8 is shown in FIG. 9. The change infrequency as a consequence of the odor is clearly seen. Due to thedecrease in the conductivity of the P-type OFETs, there is an increasein the RC time constants of the inverting stages. This causes theoscillation frequency to decrease very sharply. Note that FIG. 9 depictsthe response of the circuit of FIG. 8 to an analyte “puffed” onto thecircuit from time t1 to time t2 (approximately 5 seconds). As a resultof the incident odor “puffed” onto the circuit the frequency changedfrom around 550 Hz to around 280 Hz. Thus a frequency change of nearly50% was observed. A change in the amplitude of the oscillations was alsoobserved (i.e., change in Vmax). The observed change is also muchgreater than the change observed in a discrete OFET in response to thesame odor intensity. This demonstrates that a circuit of the type shownin FIG. 8 is a better odor/gas sensor than using a circuit using asingle OFET.

Referring to FIG. 9, it is also seen that beginning at time t2, afterthe odor (analyte) is no longer applied to the circuit, the circuitbegins to return to its condition prior to application of the odor(analyte). Applying an alternating signal to the gate of an OFET havinga polarity to turn-it-on harder for a first time period and then havinga polarity to turn-it-off for a second period of time, tends to enhancethe recovery of the OFET to the state it had prior to the application ofan analyte. This is in sharp contrast to the response of the discreteOFET shown in FIG. 2B, where the response of the discrete OFET does notbegin to recover immediately after removal of the odor (analyte).

When the ring oscillator circuit of FIG. 8 is exposed to a selectedanalyte, the mobility of the material (DDα6T) is changed and theoscillation frequency changes. This provides a convenient means ofmeasuring the presence and concentration of the analyte. By making theOFETs sensitive to certain particular analytes and not to others it isalso possible to ascertain the presence of these certain analytes.However, usually odorant detection will be done by pattern recognitionbased upon the responses of several sensors. Therefore, it is generallynot necessary to have sensors that respond only to a particular odorant.

In the circuit of FIG. 8 the P-type transistors P1-P5 are OFETs formedon an integrated circuit (IC) by similar masking and processing steps.It is possible to obtain a still higher gain response by using OFETs ofcomplementary conductivity as shown in FIG. 10. FIG. 10 is anotherembodiment of an oscillator circuit in which complementary inverters arearranged such that in every other inverter (e.g., the odd numberedinverters) the P-type transistor is an odor-sensitive OFET and in theintermediate inverters the N-type transistor is an odor-sensitive OFET.The OFETs in the circuit of FIG. 10 are formed of materials which causetheir conductivity to decrease when a selected analyte is puffed on theOFETs. Consequently, when an analyte is applied to the ring oscillatorcircuit of FIG. 10, the conductivity of OFETs P1, P3 and P5A and OFETsN2A and N4A decreases. Therefore, each cascaded inverter is responsiveto the presence of the analyte. In addition, the output of each inverter(e.g., I1) is applied to the input of the next inverter (e.g., I2) alongthe chain with a phasal relationship that results in the furtheramplification by the next inverter (e.g., I2) of the signal from thepreceding stage (e.g., I1). For example, beginning with inverter I1, inresponse to an odor signal, the output of inverter I1 produces a signalwhich is an amplified version of the response of OFET P1. Since theconductivity of P1 decreases, in response to the odor, the effectiveimpedance of P1 increases and the current through P1 decreases resultingin more time being required to charge the capacitance at the output nodeof inverter I1. The output of I1 is applied to the input of inverter I2.By making N2 an OFET whose conductivity also decreases (i.e., itseffective impedance increases) in a similar manner to that of OFET P1,inverter I2 functions to further amplify the response at the output ofI1. This is evident from noting that as the effective impedance of OFETN2 increases it causes the voltage at the output of I2 to be dischargedmore slowly and hence the output of I2 to decrease more slowly from itshigh state. Concurrently, the decrease in the voltage at the output ofI1 applied to the gate of N2 also causes N2 to conduct less. Hence, thecondition at the output of I2 is reinforced by the signal at the outputof I1. In a similar manner to that just described, making P3 an OFET andN3 a regular FET ensures that the signals from the previous stages isamplified in phase with the signal generated by P3 in response to itssensing an analyte. This same amplification of the sensed signal withina stage in cascade with the amplified signals of the previous stagesoccurs in inverter I4.

Different forms of the cascaded inverting stages using OFETs to senseodors of the type shown in FIGS. 8 and 10 may be used to practice theinvention. An embodiment shown in FIG. 10A includes a first invertingstage comprising a P-type OFET, T1, and an odor-insensitive FET, T2. Thesource electrode of T1 is connected to power terminal 81 to which isapplied VDD volts and its drain electrode is connected to node 101. Abias voltage VB is applied to the gate of T1 to bias T1 at a desiredoperating point. T2 is shown as an N-type FET, but it may be a P-typeFET or any load device. A control voltage, VC1, is applied to the gateof T2 to control the conductivity of T2 independently of the biasvoltage applied to T1. The drains of T1 and T2 are connected in commonat node 101 at which is produced the output of the first invertingstage. The source of T2 is returned to ground potential. The secondinverting stage includes a P-type OFET, T3, having its gate electrodeconnected to node 101, its source electrode connected to node 81 and itsdrain connected to node 103. A load, shown as a resistor RL, but which,in practice, may be a passive or active load, is connected between node103 and ground potential. The signal generated at node 103 may besupplied to any suitable signal amplifying or processing circuit.

Another embodiment is shown in FIG. 10B. the first inverting stage issimilar to that of FIG. 10A. However, the second inverting stageincludes an N-type OFET T3A connected at its gate electrode to node 101,at its source electrode to ground potential and at its drain electrodeto output node 103. A fourth transistor T4 has its source-to-drain pathconnected between terminal 81 and node 103. the gate electrode of T4 isshown connected to a control voltage source VC2 designed to control theconductivity (impedance) of T4. T4 may be an active load (e.g., anN-FET, a P-FET, or an OFET) or it maybe replaced by a passive resistiveload.

In FIGS. 10A and 10B, the inverting stages are cascaded to enhancesignal amplification and increase the sensitivity of the odor sensitivetransistors to the application of analytes (odors). The circuits ofFIGS. 8, 10, 10A and 10B illustrate the use of multiple sensors (two, ormore OFETs) that are connected in circuit to coherently amplify theeffects of an analyte by acting synchronously. Thus, small changesproduced in a single stage in response to a weak analyte concentrationapplied to the circuit are amplified over several stages leading to animprovement in signal to noise.

Referring to FIG. 11, there is shown an oscillator 110 coupled to acounter circuit 111 whose outputs are coupled to an alarm circuit 113.The oscillator 110 may be any suitable oscillator using at least oneodor-sensitive OFET for varying the frequency of oscillation in responseto a selected odor. By way of example, oscillator 110 may be a ringoscillator as shown in FIGS. 8 and 10. Any suitable output signal of theoscillator can be applied to a counter 111 that tracks and calculatesfrequency of oscillations. If the frequency decreases below apredetermined level, for the condition where the response of the OFET toa selected odor causes the oscillator to decrease, the output of thecounter 111 activates processing circuitry 113 and activates an alarm.Alternatively, if the frequency increases above a predetermined level,for a condition when the response of the OFET to a selected odor causesthe oscillation to increase., the output of the counter 111 activatesprocessing circuitry 113 and activates an alarm.

OFETS for use in circuits embodying the invention may be formed as shownin FIG. 12. Note that a substrate 120 of standard Si electronics (bothFET and bipolar) fabricated in a conventional manner known in the art asintegrated circuit (IC) fabrication may be used. After the fabricationof the different levels of metallization needed for the Si circuitry,organic transistor sensor circuits are fabricated employing anupside-down approach. In the upside down approach, OFET circuits areformed by sequentially defining the interconnects, the gate metal level,a dielectric layer, source-drain metal level and the organicsemiconductor layer. The organic semiconductor sees minimalpost-deposition processing. The approach of FIG. 12 is different fromtypical circuit fabrication procedures where the transistor devices arefirst formed followed by the interconnections. However, any suitablefabrication scheme may be used to form circuits embodying the invention.

In FIG. 12, the fabrication of the organic FET circuits begins with thedeposition (above the silicon circuitry) of a thick layer (122, 124) ofSiO₂(for isolation). The metal lines and vias (through-holes for themetal interconnects) are defined by photolithography and standardsemiconductor processing techniques. The interconnection metal (AI)level is defined immediately above the substrate followed by the gatemetal level (AI) and the source/drain metal level. The gate dielectric124 may be a bilayer consisting of 20 nm of Si₃N₄ and 10 nm of SiO₂,with a capacitance of 200 nF/cm². The interlayer isolation dielectrics122 are SiO₂ or Si₃N₄. The organic semiconductors are deposited as athin layer above and between the source (S) and drain (D) contacts. TheS/D contacts are coated with a gold layer by electroless/immersiontechniques to facilitate good ohmic contact with the organicsemiconductors. The underlying Si electronics and the above-lyingorganic circuitry are electrically interconnected as required throughdielectrics by forming vias. The organic transistor circuitry mayinclude any combination of the circuits described above. The activeorganic semiconductor material is deposited over the pre-formedsource-drain contacts and gate insulator may be any suitable materialfor the desired sensor selectivity.

The molecular structures of some materials used to form OFETs are shownin FIG. 13. Exemplary materials for active semiconductor layers ofP-type OFETs include:

a. didodecyl α-sexithiophene (DDα6T);

b. dioctadecyl α-sexithiophene;

c. copperphthallocyanine;

d. α-sexithiophene;

e. α,ω-dihexylsexithiophene;

f. poly(3-alkythiophene);

g. poly(3-hexylthiophene); and

h. poly(3-dodecylthiophene).

Exemplary materials for active semiconductor layers of N-type OFETsinclude:

a. hexadecafluorocopperphthallocyanine (F₁₆CuPc); and

b. naphthalenetetracarboxylic diimide compounds.

These materials are listed by way of example only and any other suitablematerials may be used.

The molecular structures of some odors used to test circuits embodyingthe invention are shown in FIG. 14. However, it should be understoodthat any gas, chemical vapor, odor or analyte which causes a change inthe conductivity of an OFET may be sensed by circuits embodying theinvention.

The various embodiments shown herein are for purpose of illustration,and the invention may be practiced using any suitable circuit.

What is claimed is:
 1. An analyte sensor comprising: a series of Ninverters, where N is an odd integer greater than 1, each inverterhaving a signal input terminal and a signal output terminal andincluding a first transistor with source, drain and gate electrodes, thegate electrode being connected to the signal input terminal of thecorresponding inverter and the drain electrode being connected to thesignal output terminal of the corresponding inverter; wherein at leastone of the first transistors is an organic transistor having aconduction path whose conductivity is responsive to presence of an odorwith the signal output terminal of each inverter being connected to thesignal input terminal of the next inverter in the series.
 2. The analytesensor of claim 1, wherein the signal output terminal of the lastinverter of the series and the signal input terminal of the firstinverter of the series are connected to form a ring oscillator.
 3. Ananalyte sensor as claimed in claim 2, wherein each one of said firsttransistors is an organic field effect transistor (OFET) having aconduction path whose conductivity changes in response to the selectedodor incident on said first transistors.
 4. An odor sensor as claimed inclaim 3, wherein the first transistors of every other inverter are of afirst conductivity type and the first transistors of the remaininginverters are of a second conductivity type complementary to said firstconductivity type.
 5. An analyte sensor as claimed in claim 2, whereineach one of said N inverters includes a second transistor; wherein oneof said first and second transistors of each inverter is an odorsensitive organic transistor and the other one of said first and secondtransistors is odor insensitive.
 6. An analyte sensor as claimed inclaim 2, wherein each one of said N inverters includes a secondtransistor; wherein one of said first and second transistors of eachinverter is an odor sensitive organic field effect transistor (OFET) andthe other one of said first and second transistors is odor insensitivefield effect transistor (FET).
 7. An analyte sensor as claimed in claim2, wherein each one of said N inverters includes a second transistor;wherein one of said first and second transistors of each inverter is anodor sensitive organic transistor and the other one of said first andsecond transistors is an odor insensitive organic transistor.
 8. Ananalyte sensor as claimed in claim 3, wherein said organic field effecttransistor has a semiconductor channel formed from one of the followingmaterials: (a) didodecyl α-sexithiophene (DD α6T); (b) dioctadecylα-sexithiophene (c) copperphthallocyanine; (d) α-sexithiophene; (e)α,ω-dihexylsexithiophene; (f) poly(3-alkythiophene); (g)poly(3-hexylthiophene); (h) poly(3-dodecylthiophene); (i)hexadecafluorocopperrphthallocyanine (F 16CuPc); and (j)naphthalenetetracarboxylic diimide compounds.
 9. The analyte sensor asclaimed in claim 5, wherein one of said first and second transistors hasa conductivity that decreases when subjected to the presence of theanalyte.
 10. The analyte sensor of claim 2, wherein the frequency ofoscillation of the ring oscillator decreases when the ring oscillator issubjected to the analyte.
 11. A gas sensor comprising: N inverters,where N is an odd integer greater than 1, wherein each one of saidinverters has a signal input terminal and a signal output terminal; eachinverter having a first transistor of first conductivity and a secondtransistor of a complementary conductivity type; the first and secondtransistors of each inverter being interconnected to form an inverter,one of the first and second transistors of one of the inverters being anorganic transistor which is responsive to an analyte; and the Ninverters being connected in a cascade, with the signal output terminalof the last inverter of the cascade being connected to the signal inputterminal of the first inverter of the cascade to form a ring oscillator.12. A gas sensor as claimed in claim 11, wherein said transistors arefield effect transistors (FETs) and wherein each organic transistor isan organic field effect transistor (OFET).
 13. The sensor of claim 11,wherein the first and second transistors are organic field effecttransistors with different sensitivities to the analyte.
 14. The sensorof claim 11, wherein the analyte is one of an odor, a chemical and avapor.
 15. A gas sensor as claimed in claim 13, wherein the firsttransistors have active channels formed from one of the followingmaterials: a. didodecyl α-sexithiophene (DDα6T); b. dioctadecylα-sexithiophene c. copperphthallocyanine; d. α-sexithiophene; e.α,ω-dihexylsexithiophene; f. poly(3-alkythiophene); g.poly(3-hexylthiophene); and h. poly(3-dodecylthiophene); and wherein thesecond transistors have active channels formed from one of the followingmaterials: hexadecafluorocopperphthallocyanine (F₁₆CuPc); andnaphthalenetetracarboxylic diimide compounds.
 16. An apparatuscomprising: an odor-sensitive organic transistor having a semiconductorchannel exposed to ambient odors; and an odor-insensitive organictransistor having a semiconductor channel protected from ambient odors,the two transistors being interconnected together to generate an outputsignal dependent on currents flowing through the two transistors and onwhether an ambient odor is present.
 17. An apparatus as claimed in claim16 wherein the two transistors are of complementary conductivity typeand are interconnected to form a complementary inverter.
 18. Anapparatus as claimed in claim 17 wherein the output signal is generatedby the inverter and the inverter has a switching point dependent on bothtransistors and on whether the odor is present.
 19. A gas sensorcomprising: a ring oscillator including at least one organic fieldeffect transistor (OFET), the ring oscillator having frequency ofoscillation that is a function of whether at least one of selected odorsis present; and a circuit responsive to the frequency of oscillation ofthe ring oscillator and configured to provide an indication of when theoscillations of the oscillator are within or outside a predeterminedrange.
 20. The sensor of claim 19, wherein the oscillator is a cascadedchain of inverters, the inverters including odor-sensitive organictransistors.
 21. An odor sensing circuit including: an organic fieldeffect transistor (OFET) having source and drain electrodes defining theends of a conduction path and a gate electrode; the conductivity of theconduction path of said OFET changing in response to selected odorsincident on the OFET and in response to the value of voltages applied tothe gate electrode; circuitry for biasing the OFET so it is responsiveto the application of odors; and circuitry for applying an alternatingsignal to the gate of the OFET for enhancing its recovery to thecondition existing prior to the application of any odors.
 22. An analytesensor comprising: first and second power terminals for the applicationof an operating potential therebetween; first and second organic fieldeffect transistors (OFETs) responsive to the presence of an analyte,each one of said OFETs having source, drain and gate electrodes; thesource and drain of the first OFET being connected between said firstpower terminal and a first node; the gate electrode of the second OFETbeing connected to the first node; and the source and drain of thesecond OFET being connected between a second node for producing thereata signal indicative of the presence of said analyte and one of saidfirst and second power terminals.
 23. An analyte sensor as claimed inclaim 22, wherein said first and second OFETs are of the sameconductivity type and wherein the source of the second OFET is connectedto said first power terminal.
 24. An analyte sensor as claimed in claim23 wherein a bias voltage is applied to said first OFET.
 25. An analytesensor as claimed in claim 24, wherein a third transistor is connectedbetween said first node and the second power terminal, and wherein anoutput load is connected to the second node.
 26. An analyte sensor asclaimed in claim 22, wherein said first and second OFETs are ofcomplementary conductivity type and wherein the source of the secondOFET is connected to said second power terminal.
 27. An analyte sensoras claimed in claim 26, wherein a bias voltage is applied to said firstOFET; and wherein a third transistor is connected between said firstnode and said second power terminal and a fourth transistor is connectedbetween the second node and the first power terminal.